java.io
Class PushbackInputStream

A PushbackInputStream adds
functionality to another input stream, namely
the ability to "push back" or "unread"
one byte. This is useful in situations where
it is convenient for a fragment of code
to read an indefinite number of data bytes
that are delimited by a particular byte
value; after reading the terminating byte,
the code fragment can "unread" it, so that
the next read operation on the input stream
will reread the byte that was pushed back.
For example, bytes representing the characters
constituting an identifier might be terminated
by a byte representing an operator character;
a method whose job is to read just an identifier
can read until it sees the operator and
then push the operator back to be re-read.

PushbackInputStream

Creates a PushbackInputStream
with a pushback buffer of the specified size,
and saves its argument, the input stream
in, for later use. Initially,
there is no pushed-back byte (the field
pushBack is initialized to
-1).

read

Reads the next byte of data from this input stream. The value
byte is returned as an int in the range
0 to 255. If no byte is available
because the end of the stream has been reached, the value
-1 is returned. This method blocks until input data
is available, the end of the stream is detected, or an exception
is thrown.

This method returns the most recently pushed-back byte, if there is
one, and otherwise calls the read method of its underlying
input stream and returns whatever value that method returns.

read

Reads up to len bytes of data from this input stream into
an array of bytes. This method first reads any pushed-back bytes; after
that, if fewer than len bytes have been read then it
reads from the underlying input stream. This method blocks until at
least 1 byte of input is available.

unread

Pushes back a portion of an array of bytes by copying it to the front
of the pushback buffer. After this method returns, the next byte to be
read will have the value b[off], the byte after that will
have the value b[off+1], and so forth.

Parameters:

b - the byte array to push back.

off - the start offset of the data.

len - the number of bytes to push back.

Throws:

IOException - If there is not enough room in the pushback
buffer for the specified number of bytes.

Since:

JDK1.1

unread

Pushes back an array of bytes by copying it to the front of the
pushback buffer. After this method returns, the next byte to be read
will have the value b[0], the byte after that will have the
value b[1], and so forth.

Parameters:

b - the byte array to push back

Throws:

IOException - If there is not enough room in the pushback
buffer for the specified number of bytes.

Since:

JDK1.1

available

Returns the number of bytes that can be read from this input stream
without blocking. This method calls the available method
of the underlying input stream; it returns that value plus the number of
bytes that have been pushed back.

skip

Skips over and discards n bytes of data from this
input stream. The skip method may, for a variety of
reasons, end up skipping over some smaller number of bytes,
possibly zero. If n is negative, no bytes are skipped.

The skip method of PushbackInputStream
first skips over the bytes in the pushback buffer, if any. It then
calls the skip method of the underlying input stream if
more bytes need to be skipped. The actual number of bytes skipped
is returned.